1. Field of the Invention
The present invention relates to a printing apparatus and a printing control method. Particularly, the present invention relates to, for example, a printing apparatus which performs printing by accurately controlling the reciprocal scanning position of a carriage with an inkjet printhead, and a printing control method.
2. Description of the Related Art
A printer which prints desired information such as a text or image on a sheet-like printing medium such as a paper sheet or film is widely used as an information output device for, for example, a wordprocessor, personal computer, or facsimile apparatus.
Various printing methods are known. An inkjet method has particularly received a great deal of attention in recent years because it is quiet and capable of facilitating color printing and performing noncontact printing on a printing medium such as a printing paper sheet. An inkjet printer generally employs a serial printing method that is inexpensive and easily enables size reduction. In this method, printing is performed in accordance with desired print information while reciprocally scanning an inkjet printhead provided on a carriage in a direction perpendicular to the conveyance direction of a printing medium.
Along with the recent improvements of printing technologies, high-resolution serial printers have been introduced commercially. In such a high-resolution printer, the accuracy of position information of the carriage moving direction (main scanning direction) badly affects the printing quality.
Regarding the printer performance, not only the printing resolution but the printing speed is also required to be higher. To meet this requirement, high-speed high-resolution printers have been commercialized.
For faster printing, it is necessary to increase the moving speed in the main scanning direction. However, the higher the speed becomes, the more the accuracy of position information necessary for high-resolution printing deteriorates.
Printers using a so-called encoder to acquire accurate position information are commercially available. The encoder is designed to output the index of an absolute position in the main scanning direction of a carriage with a printhead, and for example, an optical encoder is well known.
According to arrangement of a general optical encoder, a reference (scale) having slits at very narrow intervals (predetermined intervals) is fixed along with the main scanning direction on the printer main body. A sensor provided on the carriage reads a slit, and detects the carriage moving position and speed based on the sensor output signal. The slits normally serve as printing position indices and are provided on the scale at predetermined intervals as position information (space information).
The intervals (position resolution) of the slits preferably match the actual printing resolution (printing intervals). However, when the printing resolution rises, as described above, it is necessary to manufacture a scale corresponding to the resolution and improve the sensitivity of the sensor to read information from the scale, resulting in an increase in the cost of the encoder.
To cope with high-resolution printing using an inexpensive encoder, a scale with a resolution lower than the actual printing resolution is used. Printing position information at a higher resolution is generated by interpolation based on slits formed in the scale at predetermined intervals. The carriage moving position is obtained in accordance with the printing position information, and driving of the printhead is controlled. In this case, to ensure the accuracy of printing position information, the printing region is set only within the constant-speed moving region of the carriage.
FIG. 7 is a graph showing the relationship between the carriage moving speed and the time.
Referring to FIG. 7, the abscissa represents the time, and the ordinate represents the carriage moving speed. As shown in FIG. 7, the total time required for carriage movement is B. The time of movement in the constant-speed region is A. Hence, of the time B of carriage movement, only the time A is usable for printing. The time (B−A) (acceleration/deceleration time) is unwanted for printing.
This also badly affects the size of the printer main body. More specifically, since the region for acceleration/deceleration of the carriage is necessary in the carriage scanning direction in addition to the printing region, the size of the printer in the carriage scanning direction becomes large.
To shorten the printing time, it is necessary to increase the speed in the constant-speed region and shorten the time required for acceleration/deceleration. In this case, however, the carriage must be accelerated/decelerated abruptly, and for this purpose, large kinetic energy must be given to the carriage.
To supply large kinetic energy, the driving mechanism including a carriage motor needs to be more durable. In addition, the power consumption of the driving mechanism increases. As a result, the driving mechanism becomes bulky and expensive, and this is disadvantageous for power consumption.
To solve the above problem, Japanese Patent Laid-Open No. 2002-277231 discloses an invention directed to provide a movement control apparatus, printing apparatus, and movement control method capable of accurately controlling the current position of a carriage even during acceleration/deceleration.
An apparatus disclosed in Japanese Patent Laid-Open No. 2002-277231 has the following components: a sensor which is attached to a carriage moving along a scale with a plurality of slits provided at predetermined intervals and detects a slit, a means for predicting the time until next slit detection based on the sensor output, and a means for generating a signal related to the current carriage position based on the predicted time.
FIG. 8 is a block diagram showing the carriage position detection unit in the apparatus disclosed in Japanese Patent Laid-Open No. 2002-277231.
Referring to FIG. 8, a scale 101 has a plurality of slits at predetermined intervals. An encoder sensor 102 detects a slit of the scale 101. The encoder sensor 102 includes a light-emitting element 121 which irradiates the scale 101 with light, and a light-receiving element 122 which receives light that has passed through the slits of the scale 101. A detection unit 103 detects an output signal from the encoder sensor 102. An acceleration/deceleration prediction unit 104 predicts acceleration or deceleration of the carriage based on the detection signal from the detection unit 103 and predicts the time from the current detected slit to the next slit.
A printing position generation unit 105 generates a printing position signal of the printhead. A prediction interpolator 106 obtains position information at a resolution equal to or more than that of the scale 101 by interpolation based on an output signal (Tx) from the acceleration/deceleration prediction unit.
The detection unit 103, acceleration/deceleration prediction unit 104, printing position generation unit 105, and prediction interpolator 106 operate based on a clock signal (CLK) supplied to them.
As described above, the apparatus disclosed in Japanese Patent Laid-Open No. 2002-277231 predicts the time until the next slit detection. Hence, acceleration/deceleration printing can be performed more satisfactorily than before.
In the above prior art, however, the printing position generation unit generates the signal related to the current carriage position depending on only the predicted time. Hence, the accuracy of the current position information is poor.
More specifically, the carriage accelerates/decelerates even during the time up to the predicted time. According to Japanese Patent Laid-Open No. 2002-277231, the carriage positions up to the predicted time are obtained by equally dividing the predicted time. For this reason, no information that reflects the accelerated motion in a very small region is obtained.
If the acceleration of the carriage is small, the conventional prediction approximately poses no problem. However, when the carriage moving speed increases, and the printing resolution becomes higher, the accelerated motion in the very small region is also not negligible.